Re: [PATCH] hwmon: (aspeed-pwm-tacho) Using falling edge.

[Guenter Roeck <linux@roeck-us.net>]

On 6/28/21 10:26 PM, Billy Tsai wrote:
> On 2021/6/28, 9:56 PM, "Guenter Roeck" <groeck7@gmail.com on behalf of linux@roeck-us.net> wrote:
> 
>      On 6/24/21 8:35 PM, Billy Tsai wrote:
>      >   > On 2021/6/24, 8:44 PM, "Guenter Roeck" <groeck7@gmail.com on behalf of linux@roeck-us.net> wrote:
>      >   >
>      >   >      On Thu, Jun 24, 2021 at 11:58:21AM +0800, Billy Tsai wrote:
>      >   >      >> The tach shouldn't use both edges to measure. When the tach input
>      >   >      >> duty cycle isn't 50% the return value will inaccurate.
>      >   >      >>
>      >   >      > A tachometer doesn't have a duty cycle. A pwm has a duty cycle, but that
>      >   >      > is completely independent of the pwm duty cycle used to set the fan speed.
>      >   >      > So this patch does not really make sense with the above explanation.
>      >   >
>      >   > The duty cycle means the waveform that reported from the fan tach pin not pwm signal.
>      >   >
>      >   >      > The impact of this patch is likely that the reported fan speed is reduced
>      >   >      > by 50%. It may well be that the driver currently reports twice the real fan
>      >   >      > speed. I have no idea if that is the case, but if it is it should not be
>      >   >      > conditional. The description above states "when the tach input cycle isn't
>      >   >      > 50%", suggesting that this is conditional on some other configuration.
>      >   >      > I don't know what that might be either.
>      >   >
>      >   > According to the tach mode, our tach controller will sample the time of once conditional meet and translate it to tach value.
>      >   > When the tach signal duty cycle isn't 50%, using both edges mode will get the tach value with error rate.
>      >   > In addition, the current report value of both edges will twice the result which will enlarge the error rate.
>      >   > Actually, the tach signal won't be a complete 50% duty cycle, so both edges mode isn't recommanded for the fan usage.
>      >   > With rising-to-rising mode the skew time of tach signal will also effect the accuracy.
>      >   > Thus, using the falling-to-falling mode is the better way for a fan tach monitor.
>      >   > But for flexibility, I think using dts property to control the tach mode is better the user can change the mode to adapter the monitor device.
>      >   >
> 
>      >   Trying again, using my own words.
> 
>      >   A fan normally provides two short pulses per revolution. Those are short
>      >   puleses, and one does not typically talk about "duty cycle" or "waveform"
>      >   in this context. The driver currently counts both edges of those pulses.
> 
> Our tach controller will count how many tach clocks in those shot pulses.
> In both edge mode the counting period only half of the pulse. Thus, it is more sensitive
> to the signal quality of the shot pulse when using both edges mode.
> 
>      >   Assuming that a fan reports, say, 1,000 pulses per minute, the hardware
>      >   would report a edle count of 2,000. This should translate into 500 RPM.
>      >   I don't know if this is currently the case in the driver; if not, it would
>      >   be a bug. Either case, the suggested change would reduce the pulse count
>      >   reported by the hardware to 1,000. If we assume that the driver currently
>      >   translates this correctly to 500 RPM, the suggested change would result
>      >   in the driver reporting 250 RPM, which would be wrong.
> 
>      >   So there are two possibilities:
>      >   1) The driver currently reports 1,000 RPM in this situation. This would be a bug
>      >       which needs to get fixed.
>      >   2) The driver currently correctly reports 500 RPM. In this case, the suggested
>      >       patch would introduce a bug because the code is not adjusted for the reduced
>      >       pulse count.
> 
>      >   The problem is that the patch does not address either of the situations above.
>      >   In case 1), it should state that the code currently reports twice the real
>      >   fan speed, and that the patch fixes that problem. In case 2), the patch should
>      >   also fix the arithmetic used to calculate RPM from the pulse count.
>      >   Either case, I disagree that this should be handled in devicetree. It has
>      >   nothing to do with hardware description or configuration but is in the
>      >   discretion of the driver author/implementer.
> 
> The driver doesn't have the two situations you describe, it already considers the different
> sampling modes at the arithmetic. The patch is used to make users have the option to select
> the mode not just fix it to the both edges mode.
> 

Thanks for the explanation. Please include that in the patch description, and please avoid
unusual terms such as "duty cycle" or "waveform".

Thanks,
Guenter